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Revision as of 14:18, 12 September 2016
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Inflammatory Bowel Disease
Inflammatory bowel disease (IBD) describes the chronic inflammations of parts of the intestine and is a collective of several further specified illnesses. The most common conditions are ulcerative colitis and Crohn's disease. It is classified as an autoimmune disease for which no cure has been developed. Current treatments include immunosuppression, surgery, antibiotics and nutritional therapies. This disease is a severe burden for the patients as well as it causes increasing direct (treatment) and indirect (absenteeism from work) costs for society. Furthermore, only in Europe 2.5 million people are affected by IBD and the number of cases is increasing world-wide1.
Unfortunately there are no characteristic blood markers to distinguish the different forms of IBD. The diagnosis relies mostly on the location of inflammation observed during a colonoscopy. Also the underlying trigger of the disease is not completely understood but correlation studies proposed factors such as diet, genetic predisposition, breach of the intestinal barrier and unfavorable alteration of the microbiota, called dysbiosis2. The diversity of the microbiota is noticeably reduced2,2 in IBD patients and the composition of the gut flora changes from symbiotic to predominantly pathobiotic microbes1.
The inflammation of the intestine partially interrupts the integrity of the layer of epithelial cells lining the intestine. This cell layer separates the gut lumen containing trillions of microbes from the body. The damage to this essential barrier compromises the selectivity and allows for penetration of immunogenic antigens from the lumen across the epithelial layer1, which enhances the inflammation reaction.
Sensing of Markers for IBD
Nitric Oxide
Figure 1: NorR binding
Beside the penetration of immunogenic antigens across the epithelial layer, there is also non-normal leakage of inflammation markers into the gut lumen. One of these molecules is nitric oxide (NO•, t1/2 < 6 seconds1) and is one of the molecules we are going to sense with our system. The sensing of NO• with E. coli has already been described by Archer et al.1 in 2012. This work provides us with the relevant genetic elements and helps us to design this system for our purpose. Additionally, they present their system as a rapid detection system for IBD related disease flare-ups which would allow for an immediate intervention.
NorR is capable of binding NO• with its mononuclear non-heme iron center. While other sensor proteins are not only specific for NO• but also for other NOs species, NorR binds specifically the NO• radical. NorR is constitutively bound as a hexamer upstream of the norVW promoter but inhibiting transcription in absence of NO•. Once NO• binds to NorR, its ATPase activity is triggered and provides energy to form a productive interaction with the σ54 - RNA polymerase holoenzyme1.
N-Acyl Homoserine Lactones
Figure 2: DNA- and 3-oxo-C6-HSL bound, dimeric form of TraR, a close homolog of EsaR (PDB: 1L3L, edited with UCSF Chimera1)
In addition to a general inflammation marker we want to sense molecules secreted by the microbiota in order to identify the bacteria. One well-known class of molecules secreted by many bacterial species belongs to the quorum sensing (QS) system. QS molecules act as bacterial hormones among and between species which control for example the formation of biofilms and growth behaviour. Furthermore, QS molecules can alter the microbiota's composition1. The best known subclass of QS molecules are the N-acyl homoserine lactones (AHL) which will be identified by our living biosensor.
One of the AHLs to be found upregulated in IBD1 is 3-hydroxy-hexanoyl-HSL (3-OH-C6-HSL). A well characterized regulatory protein that senses a very similar HSL (3-oxo-C6-HSL) is EsaR that was used by a previous iGEM team. The special feature of EsaR is its regulatory behaviour: while most HSL-responsive elements are inducible activators, EsaR is a repressor that dissociates from the DNA in presence of HSL. This is important for our circuit as a repressor is thought to be less leaky than an activator.
As our target HSL is not the natural ligand for EsaR, we applied a directed evolution strategy to change its specificity.
Associative Learning Circuit
Overview
To serve as a diagnostics and research tool, our system should not only be able to sense a single molecule alone but should associate an inflammation marker - in our case NO• - with a potential trigger of the inflammation itself. Thus, we implemented an associative learning circuit that allows for the detection of the temporal and spatial presence of two markers.
Nitric oxide and 3-OH-C6-HSL are only two possbile markers of IBD. There exist many more that are definitively worth to be further investigated and are ideally observed in parallel. This is why we extended the AND-gate by a learning component. While the number of distinguistable reporters (e.g. fluorophores) is limited, our system allows for simultanious observation of a multitude of parallel measured markers. Our Pavlov's Coli learn the occurence of the presence of two markers and store this information in their DNA until readout.
We designed our system in a way that allows fast and easy demultiplexing of a complex mixture of different reporter strains. If the reporter strains encounter again the with inflammation associated marker, they generate an easily observable output: fluorescence. This was achieved by integrating a second AND-gate that relies on the successful learning process.
Biological Implementation: Recombinase
Sensor AND-gate:
At an inflammation spot, nitric oxide activates NorR and triggers the transcription of the bxb1 integrase gene. The transcription can only proceed if 3-OH-C6-HSL is present. The HSL lets the repressor EsaR dissociate from the regulatory element (esaBox) on the DNA and thus annihiliates its roadblock activity.
Learning:
The bxb1 integrase gene is flanked by attP and attB sites which allow an unidirectional recombination as the newly generated recombination sites are not recognized by Bxb1 anymore. Once the bxb1 gene is successfully transcribed and translated, Bxb1 binds to the attP and attB recombination sites and inverts its own gene. Now, there is no promoter in front of the bxb1 gene anymore that could further produce Bxb1.
Reporter AND-gate:
Together with the bxb1 gene, a constitutive promoter was also inverted, now being placed upstream of the reporter protein GFP. Beside the constitutive activity of this promoter, the transcription is further controlled by another esaBox, the binding site of EsaR.
After the system now has learnt to respond to the associated stimulus alone, the expression of GFP can easily be induced by just exposing it to the stimulus again, e.g. EsaR's ligand.
Biological Implementation: CRISPR/Cpf1
An alternative to a recombinase-based switch is the usage of the 2015 characterised CRISPR/Cpf1 system. Instead of cutting both DNA strands at the same position, Cpf1 cuts the DNA with an offset of four or five nucleotides, thus producing single-stranded overhangs. It is suggested that this is advantageous for genome editing via non-homologous end-joining.
We will use the features of Cpf1 to create an AND-gate controlled one-way switch with finally the same functionality as the recombinase-based switch.
For this, a reporter construct is stably integrated into the genome of E. coli whereas Cpf1 and its guide RNAs will be expressed from a plasmid.
Sensor AND-gate:
At an inflammation spot, nitric oxide activates NorR and triggers the transcription of Cfp1. The transcription can only proceed if 3-OH-C6-HSL is present. The HSL lets the repressor EsaR dissociate from the regulatory element (esaBox) on the DNA and thus annihiliates its roadblock activity. The guide RNAs are expressed constitutively at a high level.
Learning:
Once Cfp1 is expressed, it is brought to the cutting-sites by the two distinct guide RNAs. There, Cfp1 cuts out the mNectarine gene while creating sticky ends. These will then be ligated by endogeneous ligases by NHEJ which reconstitutes the GFP gene.
Reporter AND-gate:
The reconstituted GFP gene is now under the control of a constitutive promoter regulated be an esaBox, the binding site of EsaR.
After the system now has learnt to respond to the associated stimulus alone, the expression of GFP can easily be induced by just exposing it to the stimulus again, e.g. EsaR's ligand.
Directed Evolution of EsaR
Potential Delivery Method of Reporter Strains
The idea to administer genetically modified bacteria in the context of IBD was published by Steidler et al. in 20001. Generally, to use engineered probiotic bacteria as a delivery vector for in vivo produced therapeutic agents has been described multiple times1,1,1,1.As the stomach and the gastrointestinal tract are rough environments for non-adapted (probiotic) bacteria, we suggest to encapsulate the bacteria in a hydrogel. This protects the bacteria and ensures the recovery of the reporter strain. The method of encapsulation is well known for oral administration in animal models (e.g. Prakash et al.1) and is summarised in several reviews (e.g. Prakash et al. (2008)1 and Tomaro et al. (2012) 1).
References:
Thanks to the sponsors that supported our project:
